Pulling it back and forward
I bought a Kaplan pump (1.75” for about 60 bucks). After a few minutes in the pump it’s easy to lose the “natural” erection but difficult to stimulate to get hard again. I found pulling the pump back and forth whilst at pressure gives a pretty good feel, as long as your dick is touching the cylinder, and can get you hard quite nicely. Obviously you have to watch out to avoid pressure loss - and also too high pressure, since you temporarily increase the vacuum.